Voltage detector for shielded electrical conductors



Jme 27, 1967 A. M. LOKiE ETAL 3328690 VOLTAGE DETEGTOR FOR SHIELDED ELECTRICAL CONDUCTORS Filed Feb. 14. 3,964

WITNESSES I r 'NVENTO Arthur M. Lo 'e Jim ,w I John E' Cotfon United States Patent O 3,328,690 VOLTAGE DETECTOR FOR SHIELDED ELECTRICAL CONDUCTORS Arthur M. Lockie and John F. Cottor, Hickory Township,

Pa., assignors to Westinglouse Electric Corporation,

East Pittsburgh, Pa., a Corporation of Pennsylvania Filed Feb. 14, 1964, Ser. No. 344,902 1 Claim. (Ci. 324-122) This invention relates in general to voltage detectors and more particularly to voltage detectors for shielded cable.

There has been a rapid increase in the use of shielded cable in electrical power distribution systems, both overhead and underground. Cable terminations have been developed which continue the cable shielding to the case of the apparatus to which the cable is connected. This prevents accidental contact with dangerous potentials, but presents a problem when it is necessary to disconnect the shielded cable from the apparatus for maintenance, repair, or other changes in the system. In most instances, for safety or other reasons, it is undesirable to connect or disconnect cables while they are energized, and most electric utility companies require a check to make sure that a cable is deenergized before any work is performed. In open-wire Construction, or where cable is terminated in such a way that an ener gized terminal is exposed, it is common practice to make this check by placing a gasdischarge type tube on an insulating rod, and place the gas-discharge tube in the vicinity of the exposed terminal. If the terminal is energized, the electric field gradient surrounding it ionizes the gas in the tube, causing it to glow. Absence of glow is considere-d to be sufiicient evidence that the terminal is not energized.

With shielded cable, the electric field is confined between the conductor and the grounded outer shield, and the electric field around the terminal or terminals is also 'contained within a shield. Thus, the standard method of checking for the existence of potential is not suitable.

Another problem which a'ises with the usage of shielded cable is the fact that the outer shield may be formed of a -conducting, rubber-like material. Since rubber is normally associated with an insulator, it is not apparent to the eye whether or not the cable shield is actually grounded. It would be desirable to make it visibly apparent when the cable shield is in fact grounded.

Accordingly, it is an object of this invention to provide a new and improved voltage detector.

Another object of this invention is to provide a new and improved voltage detector for shielded cable installations which continue the shielding to the casing of the apparatus to which the cable is connected.

A further object of this invention is to provide a new and improved voltage detector for shielded cable which automatically exhibits a visual signal while the cable is energized with an electric potential.

Still another object of the invention is to provide a new and improved voltage detector for shielded cable terminations which also provides means for visibly grounding the shield portion of the cable.

Briefly, the present invention accomplishes the above cited objects by providing an opening in the shield portion of the cable, usually near a cable termination point. In one embodment of the invention, a gas-discharge or glow tube is disposed in and near one end of an insulating member made of a transparent acrylic polymer, or other transparent insulating material having a high refractive index. The insulating member is disposed, glow tube end first, through the opening in the shield portion of the cable, and into a recess in`the insulation between the shield and conductor portions of the cable. The glow tube is thus disposed between the shield and conductor portions of the cable where the electric field is confined. The other end of the insulating member protrudes or extends for a short distance above the outer surface of the shield portion of the cable, and lights up when the cable is energized. This is due to the ionizing of the gas in the glow tube and the transmission of this glow or light throughout the length of the insulating member. The insulating member provides the dual function of transmitting the light to a point where it can beseen, and also maintains the insulation level which might otherwise be y adversely affected due to a removal of a portion of the insulation.

Another embodment of the invention eliminates the requirement of utilizing a highly refractive transparent insulating material, and at the same time provides a voltage detector or indicator which glows at a lower voltage and much brighter at a given voltage, than the voltage detector of the first embodment. In thi-s embodment, an opening is also provided in the shield portion of the cable, and a gas-discharge or a glow tube is placed on the surface of, or in a very slight recess of the insulation such that the glow tube is still visible. Then, an auxiliary metallic shielding member, having a plurality of perforations therein, is disposed over the glow tube. The electric field is thus completely confined, as the auxiliary shielding member continues the shield around the conductor without interruption, thus preventing disturbance of the electric field and field cancellation which may occur if a large opening in the shield portion of the cable is not covered by a conducting surface. The glow of the tube, when the cable is energized, is visible through the plurality of perforations in the auxiliary shield. Further, the auxiliary shield may provide a terminal for visibly grounding the cable shield, thus eliminating any possible doubt as to whether or not the cable shield is grounded. v

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention will be pointed out in partieularity in the claims annexed to and forming a part of this specfication.

For a better Understanding of the invention, reference may be had to the accompanying drawings, in which:

FIGURE 1 is a cross sectional view of a shielded cable termination illustrating one embodment of the invention;

FIG. 2 is a cross sectional view of shielded cable illustrating another embodment of the invention; and

FIG. 3 is a cross sectional view of shielded cable illustrating a modification of the embodment shown in FIG. 2.

Referring now to the drawings, and FIG. ,1 in particular, there is illustrated a shielded cable termination 10, comprising shielded cable 11 and an apparatus casing or housing 18. The shielded cable comprises conductor means 12, insulating means 14 disposed uniformly around conductor means 12, and shelding means 16 disposed uniformly around insulating means 14. The Construction and materials used for the manufacture of shielded cable are well known in the art. For example, the conductor means 12 may be Copper, the nsulating means 14 may be rubber of the type that is an electrical insulator, and the shielding means 16 may be rubber of the type that is a conductor of electricity. The shielding means 16 terminates at and is elcctrically connected to the casing 18 of the apparatus to which the shielded cable is connected, such as a transformer. The casing 18 should be a conductor of electricity, such as aluminum or steel. It will be noted from the Construction of the shielded cable termination 10 shown in FIG. 1 that there will be substantially no electrical field from the conductor 12 outside the shielding means 16 and casing 18. Therefore, it is difficult for maintenance personnel to determine if the shielded cable 11 and apparatus 18 are connected to a source of potential. The conventional method of placing a glow tube or gas-discharge tube near the cable termination is useless, because there are no unshielded parts which have an electrical potential applied thereto. This invention overcomes this difficulty by disposing a glow tube or gasdischarge tube 20, of the type which will glow or emit visible light when subjected to a strong electric field, between the shield 16 and conductor 12. In order to subject the glow tube 20 to an electric field of suicient strength to produce a bright glow, the glow tube 20 must be inserted into the insulating layer 14, necessitating a removal of insulation 14. This presents two problems. First, the insulation strength in this area is reduced, and second, the glow tube is disposed wholly below the outer surface of the shielded cable 11 and, therefore, difficult to observe. This embodiment of the invention overcomes both of these problems by disposing the glow tube 20 near the end of a transparent member or rod 22. Member 22 should be an electrical nsulator and have a high refractive index. For example, a transparent acrylic polymer of the type commonly known as Lucite is excellent. By disposing the glow tube 20 completely within the transparent insulating member 22, the glow tube is mechanically protected and also the transparent member 22 transmits light throughout its entire length, when the glow tube emits light, due to the high refractive index of the transparent insulating member 22. Transparent insulating member 22 is disposed is an opening in the shield 16 and insulation 14, with the portion containing the glow tube 20 located between the conductor 12 and shield 16. A portion of member 22 also projects above the outer surface of the cable 11. Member 22, being an electrical nsulator, thus replaces the insulation 14 which has been removed, rebuilding the insulation strength and preventing moisture from entering the insulation 14. Member 22 also transmits light throughout its length so that the portion of member 22 which projects above the surface of the shielded cable 11 will be visibly lighted When the glow tube 20 emits light due to a potential existing on conductor 12. The projecting portion of transparent insulating member 22 may be a rod, as shown in FIG. 1, a button, ring, or any other configuration. Thus, glow tube 20 and transparent insulating member 22 provides a continuous indication of the existence of potential on conductor 12, and maintenance personnel can determine the existence of potential at a glance. Although, the glow tube 20 and member 22 are illustrated near a shielded cable termination, which is where the indication of voltage will most generally be required, it is to be understood that the invention may be utilized anywhere in the shielded cable system to provide a voltage indication. Further, the invention may be used to indicate the presence of voltage near cable connections, or may be utilized with unshielded cables which have a metallic sheath or concentric neutral. In fact, the invention disclosed herein may be used wherever it is desirable to provide an indication of the existence of voltage between conductors which are disposed such that they are not readily accessible to other means of detection.

Another embodiment of the invention is illustrated in FIGS. 2 and 3, with like reference numerals in FIGS. 2 and 3 indicating like Components. FIGS. 2 and 3 illustrate a shielded cable 30 comprising a conductor 32, insulation 34, and a shield 36. In the embodiment shown in FIG. 1, the removing of a portion of shield 16 produced some distortion or cancellation of the electric field which necessitated the insertion of the glow tube 20 fairly deep into insulating means 14 in order to encounter an electric field of sufficient magnitude to cause the glow tube 20 to emit an easily discernible glow. This then made it imperative that the insulating qualities of insulation means 14 be restored, as well as some means to transmit the light of the glow tube out past the outer surface of the shielding means 16. The means for accomplishing these objectives were hereinbefore explained. The embodiment of the invention shown in FIGS. 2 and 3 allows a strong electric field to exist at or near the surface of the shielded cable, permitting a glow tube to be disposed close enough to the surface of the cable that light transmitting means are not required. More specifically, FIG. 2 illustrates an embodiment of the invention wherein it is only necessary to remove a portion of the shield 36, with a glow tube 38 disposed in the opening and securely held in place by electrical insulating means such as an epoxy resin 40. In order to produce an electric field at the glow tube 38 of sufcient strength to cause the glow tube 38 to glow brightly when conductor 32 has its rated potential applied thereto, an auxiliary shield 42 formed of an electrically conductive material is disposed to contact the shield 36 around the opening in said shield 36 and completely cover the opening and the glow tube 38. Thus, the auxiliary shield 42 is an electrical continuation of shield 36. Auxiliary shield 42 encloses the discharge or glow tube 38 completely within any electric field from conductor 32, and also acts as a mechanical protector for the glow tube 38. A plurality of perforations 44 in the auxiliary shield 42 allows light emtted by the glow tube 38 to be clearly visible outside the auxiliary shield 42.

Another advantage in using the auxiliary shield 42 lies in the fact that a terminal 46 may be incorporated which will allow the shield portion 36 of shielded cable 30 to be visibly grounded, as shown at 43, thus eliminating any doubt which may exist due to the appearance of some shielded cables which have a shield constructed of conductive rubber.

The glow tube 38 will generally be placed near a termination of the cable, as shown in FIG. 1, although the teachings of this invention may be applied to shielded cable at any point along its length, or to any two conductors where it is desirable to provide an indication of voltage between said conductors.

FIG. 3 illustrates a modification of the embodiment of the invention shown in FIG. 2, wherein a portion of insulation 34 is removed below the opening in shield 36, to allow glow tube 38 and auxiliary shield 42 to more closely conform to the contour of the shielded cable 30. Insulation 40 serves the dual purpose of holding glow tube 38 mechanically, as well as replacing the insulation 34 which has been removed, thus restorng the insulating qualities of the cable.

In tests made on shielded cable utilizing the two embodiments of the invention described herein, the glow tube 20 shown in the first embodiment (FIG. l) glowed brightly when the voltage on conductor 12 was in the order of 10,000 volts. The glow tube 38, shown in the second embodiment (FIGS. 2 and 3) glowed brightly when the voltage on conductor 32 was in the order of 4000 volts. These voltages, however, may be varied due to different design and placement of the various Components, but do give an indication of the remarkable increase in efficiency when using the auxiliary shield 42 as taught by the embodiment shown in FIGS. 2 and 3.

The invention shown and described herein, thus, provides a quick and reliable indication of whether or not shielded cable is connected to an electrical potential. Further, one of the embodiments of the invention additionally provides means for connecting a ground, which will remove doubt as to whether the cable shield is grounded.

Since numerous changes may be made in the above described apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative, and not in a limiting sense.

We claim as our invention:

A voltage indicator for shielded electrical cable comprising, electrical conductor means, insulating means, said insulating means being disposed around said electrical conductor means, shelding means, said shelding means being disposed around said insulating means, said shelding means having an opening therein, said insulating means having a recess starting from its outer surface coinciding with the opening in said shelding means to form a chamber having a predetermined depth, glow tube means having the characteristic of emitting light when subjected to an electric field, transparent electrical insulating means having a refractive index which enables said transparent means to transmit light, said glow tube means being disposed within said transparent means, said transparent means being partially disposed within the chamber formed by the opening in said shelding means and recess in said insulating means with a portion of said transparent means projecting above the outer surface of said shelding means, the portion of said transparent means containing the glow tube being disposed between said shelding means and said conductor means, said glow tube means emitting light which is transmitted through said transparent means when said conductor means is connected to a source of electrical potential having a predetermined magnitude.

References Cited UNITED STATES PATENTS 1,867,944 7/ 1932 Nijland 324-725 2,395,851 3/ 1946 Cork 324- 2,405,814 9/1946 Brannn 324-122 X 2,408,198 9/ 1946 Coltman 324-95 X 2,478,007 9/ 1949 Collard 324-95 2,704,348 3/ 1955 Carlin 324-95 FOREIGN PATENTS 830,073 3/ 1960 Great Britain.

WALTER L. CARLSON, Pr'mary Examner. RUDOLPH V. ROLINEC, Exam'ner. G. L. LETT, E. F. KARLSEN, Assistant Exam'ners. 

